Superabsorbent Polymer Particles Having a Reduced Amount of Fine-Sized Particles, and Methods of Manufacturing the Same

ABSTRACT

Superabsorbent polymer particles having a reduced amount of fine-sized particles and methods of producing the superabsorbent polymer particles are disclosed.

FIELD OF THE INVENTION

The present invention relates to coated superabsorbent polymer particleshaving a reduced amount of fine-sized particles. The present inventionalso relates to methods of manufacturing superabsorbent polymerparticles using a film-forming polymer or a wax to reduce the amount offine-sized superabsorbent polymer particles. The present invention alsorelates to the use of the superabsorbent polymer particles in articles,such as diapers, catamenial devices, and wound dressings.

BACKGROUND OF THE INVENTION

Water-absorbing resins are widely used in sanitary goods, hygienicgoods, wiping cloths, water-retaining agents, dehydrating agents, sludgecoagulants, disposable towels and bath mats, disposable door mats,thickening agents, disposable litter mats for pets,condensation-preventing agents, and release control agents for variouschemicals. Water-absorbing resins are available in a variety of chemicalforms, including substituted and unsubstituted natural and syntheticpolymers, such as hydrolysis products of starch acrylonitrile graftpolymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonatedpolystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols,polyethylene oxides, polyvinylpyrrolidones, and polyacrylonitriles. Themost commonly used polymer for absorbing electrolyte-containing aqueousfluids, such as urine, is neutralized polyacrylic acid, i.e., containingabout 50% to 100%, neutralized carboxyl groups.

Such water-absorbing resins are termed “superabsorbent polymers,” orSAPs, and typically are lightly crosslinked hydrophilic polymers. SAPsare generally discussed in Goldman et al. U.S. Pat. Nos. 5,669,894 and5,599,335, each incorporated herein by reference. SAPs can differ intheir chemical identity, but all SAPs are capable of absorbing andretaining amounts of aqueous fluids equivalent to many times their ownweight, even under moderate pressure. For example, SAPs can absorb onehundred times their own weight, or more, of distilled water. The abilityto absorb aqueous fluids under a confining pressure is an importantrequirement for an SAP used in a hygienic article, such as a diaper.

As used herein, the term “SAP particles” refers to superabsorbentpolymer particles in the dry state, i.e., particles containing from nowater up to an amount of water less than the weight of the particles.The term “particles” refers to granules, fibers, flakes, spheres,powders, platelets, and other shapes and forms known to persons skilledin the art of superabsorbent polymers. The terms “SAP gel” and “SAPhydrogel” refer to superabsorbent polymer particles in the hydratedstate, i.e., particles that have absorbed at least their weight inwater, and typically several times their weight in water. The term“coated SAP particles” refers to particles of the present invention,i.e., SAP particles having a water-insoluble polymer or wax coating.

The terms “fine-sized SAP particles” and “SAP fines” refer to SAPparticles having a particle diameter of about 100 microns or less. Inthe production of SAP particles, a significant portion of SAP fines,e.g., about 10 percent by weight, can be generated. The presentinvention is directed to substantially reducing the amount of SAP finesgenerated during manufacture of SAP particles.

The term “surface crosslinked” refers to an SAP particle having itsmolecular chains present in the vicinity of the particle surfacecrosslinked by a compound applied to the surface of the particle. Theterm “surface crosslinking” means that the level of functionalcrosslinks in the vicinity of the surface of the SAP particle generallyis higher than the level of functional crosslinks in the interior of theSAP particle. As used herein, “surface” describes the outer-facingboundaries of the particle. For porous SAP particles, exposed internalsurfaces also are included in the definition of surface.

The term “polymer coating” or “wax coating” refers to a coating on thesurface of an SAP particle comprising a nonreactive, water-insolublepolymer and/or wax. The film-forming polymer that coats the surface ofthe SAP particle is different from the polymeric material of the SAPparticle, and typically does not function as an SAP. The polymer and/orwax coating does not adversely affect the absorption profile of the SAPparticles, and reduces the amount of SAP fines generated during SAPparticle manufacture.

SAP particles can differ in ease and cost of manufacture, chemicalidentity, physical properties, rate of water absorption, and degree ofwater absorption and retention, thus making the ideal water-absorbentresin a difficult composition to design. Therefore, extensive researchand development has been directed to providing a method of increasingthe fluid absorption properties of SAP particles. This is a difficultgoal to achieve because improving one desirable property of an SAPparticle often adversely affects another desirable property of the SAPparticle. For example, absorptivity and gel permeability are conflictingproperties. Therefore, a balanced relation between absorptivity and gelpermeability is desired in order to provide sufficient liquidabsorption, liquid transport, and dryness of the diaper and the skinwhen using SAP particles in a diaper.

In this regard, not only is the ability of the SAP particles to retain aliquid under subsequent pressure an important property, but absorptionof a liquid against a simultaneously acting pressure, i.e., duringliquid absorption, also is important. This is the case in practice whena child or adult sits or lies on a sanitary article, or when shearforces are acting on the sanitary article, e.g., leg movements. Thisabsorption property is referred to as absorption under load.

The current trend in the hygiene sector, e.g., in diaper design, istoward ever thinner core constructions having a reduced cellulose fibercontent and an increased SAP content. This is an especially importanttrend in baby diapers and adult incontinence products.

This trend has substantially changed the performance profile required ofSAPs. Whereas SAP development initially was focused on very highabsorption and swellability, it subsequently was determined that anability of SAP particles to transmit and distribute a fluid both intothe particle and through a bed of SAP particles also is of majorimportance. Conventional SAPs undergo great surface swelling when wettedwith a fluid, such that transport of the fluid into the particleinterior is substantially compromised or completely prevented.Accordingly, a substantial amount of cellulose fibers have been includedin a diaper core to quickly absorb the fluid for eventual distributionto the SAP particles, and to physically separate SAP particles in orderto prevent fluid transport blockage.

An increased amount of SAP particles per unit area in a hygiene articlemust not cause the swollen SAP hydrogel particles to form a barrierlayer to absorption of a subsequent fluid insult. Therefore, an SAPhaving good permeability properties ensures optimal utilization of theentire hygiene article. This prevents the phenomenon of gel blocking,which in the extreme case causes the hygiene article to leak. Fluidtransmission and distribution, therefore, is of maximum importance withrespect to the initial absorption of body fluids.

Fine-sized SAP particles contribute significantly to gel blocking.Therefore, extensive effort has been expended to find methods ofreducing the generation of SAP fines during SAP manufacture. Thereduction of SAP fines is one method wherein permeability properties ofSAP particles can be improved without a concomitant reduction inabsorption properties.

As previously stated, SAPs typically are based on acrylic acid and/or asalt thereof. The SAP particles are prepared by polymerizing acrylicacid and/or a salt thereof in the presence of an internal crosslinkingagent. In general, a monomer solution containing acrylic acid and/or anacrylic acid salt, plus an internal crosslinking agent, is polymerizedto form a hydrogel. The hydrogel then is chopped and dried, followed bymilling and sieving.

Sieving separates fine-sized SAP particles from SAP particles having thedesired particle size. The fine-sized SAP particles have a particle sizeof less than about 100 μm, and are separated because SAP fines adverselyaffect SAP performance in an absorbent article. In particular,commercial applications of SAP fines are limited by the small particlesize which typically results in handling problems, such as dusting, andperformance problems, such as gel blocking.

Initially, manufacturers used the entire SAP product, including fines,in absorbent products. It was discovered, however, that the inclusion ofSAP fines lowered product performance. The difficulty occurs when thefine particles are contacted with an aqueous fluid, which results in a“gel blocking” phenomenon. Upon hydration of a tightly packed mass ofSAP fines, only the outside layer is wetted because the fines form adense network such that neither capillary action nor diffusion permitspenetration of the fluid into uniform contact with the interiorparticles. The result is a substantially reduced overall capacity of theSAP to absorb and hold aqueous fluids.

SAP fines also produce a dusting problem. Fine SAP dust having aparticle size of less than about 10 μm is undesirable for inhalationtoxicity reasons, and SAP fines smaller than 100 μm cause visuallydetectable dusting that lead to handling problems in production andprocessing operations.

The sieved fractions of SAP particles having a particle size of about100 μm or greater are admixed, then typically subjected to a surfacecrosslinking process, followed by a second sieving operation. Thesurface-crosslinking process produces additional SAP fines, and thesecond sieving operation separates additional SAP fines, i.e.,surface-crosslinked SAP fines, also having a particle size of less thanabout 100 microns.

The separated fine-sized SAP particles, including surface-crosslinkedSAP fines, typically are not used in practical applications, and eitherare discarded or recycled into a wet stage of the SAP manufacturingprocess such that their polymeric material forms a part of the finaldried SAP particles. Simply discarding SAP fines is economicallyundesirable. However, recycling of SAP fines also is costly. The once ortwice (if surface crosslinked) dried SAP fines are reintroduced earlyinto the SAP process for recycling, which hydrates the SAP fines.Consequently, the prior drying steps, including the time required andenergy costs, are wasted for the percent of SAP fines that are recycled.In addition, even after recycling, the subsequent drying and sizingsteps generate additional SAP fines that again must be reintroduced inthe SAP manufacturing process. Accordingly, the recycling of SAP finesis a continuous and costly process.

Directly recycling SAP fines into the SAP manufacturing process also hasother disadvantages, for example, providing SAP particles that resistdrying or that dry at uneconomical rates. Recycling SAP fines also oftenfails to provide an SAP particle that resists disintegration duringsubsequent processing, such as sizing, pneumatic conveying, or surfacecrosslinking, which are necessary for forming commercial SAP products.The recycling of SAP fines in this manner also is costly, and tends toreduce the gel strength of the final SAP particles.

The present invention is directed to SAP particles, optionally surfacecrosslinked, that are coated with a nonreactive, film-forming polymerand/or a wax. The methods of manufacturing these coated SAP particlesreduce the amount of fine-sized SAP particles that are generated, and,therefore, decrease manufacturing costs by reducing the amount of SAPfines that are recycled.

SUMMARY OF THE INVENTION

The present invention is directed to SAP particles having a coatingcomprising a film-forming polymer and/or a wax. More particularly, thepresent invention is directed to methods of preparing optionallysurface-crosslinked SAP particles that generate a reduced amount offine-sized particles, and to SAP particles having a coating comprising afilm-forming polymer and/or a wax.

One aspect of the present invention is to provide methods ofmanufacturing SAP particles that generate a reduced amount of fine-sizedparticles, i.e., particles less than 100 microns in size. As a result,recycling of SAP fines is substantially reduced and significant costsavings are achieved.

Another aspect of the present invention is to provide optionallysurface-crosslinked SAP particles coated with a film-forming polymerand/or wax. Coating of the SAP particles with a film-forming polymerand/or wax, and optional surface crosslinking, can be performedsimultaneously or sequentially. In the absence of a surface crosslinkingprocess, the film-forming polymer and/or wax is applied to SAP hydrogelparticles after a chopping, i.e., sizing, step and prior to or during adrying step.

Still another aspect of the present invention is to prepare coated SAPparticles of the present invention by applying less than about 0.1% of afilm-forming polymer, less than 0.05% of a wax, or both, by weight ofthe SAP particles, to surfaces of the SAP particles, followed by heatingthe resulting SAP particles at about 70° C. to 175° C. for about 5 toabout 90 minutes.

Another aspect of the present invention is to provide a method ofmanufacturing SAP particles wherein the method generates less than 2%,by weight, particles having a particle size of 100 microns or less.

Yet another aspect of the present invention is to provide absorbenthygiene articles, such as diapers, having a core comprising coated SAPparticles of the present invention.

Another aspect of the present invention is to provide absorbent hygienearticles having a core containing a relatively high concentration ofcoated SAP particles of the present invention.

These and other aspects and advantages of the present invention willbecome apparent from the following detailed description of the preferredembodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to SAP particles coated with afilm-forming polymer and/or a wax. The polymer or wax is applied to SAPparticles or a comminuted SAP hydrogel during the manufacturing process,which reduces the amount of fine-sized SAP particles generated duringsubsequent processing. Accordingly, a reduced amount of SAP fines arerecycled through the SAP manufacturing process, which results in asubstantial cost savings and SAP particles having an improved absorptionprofile.

SAPs for use in personal care products to absorb body fluids are wellknown. SAP particles typically are polymers of unsaturated carboxylicacids or derivatives thereof. These polymers are rendered waterinsoluble, but water swellable, by crosslinking the polymer with a di-or polyfunctional internal crosslinking agent. These internallycrosslinked polymers are at least partially neutralized and containpendant anionic carboxyl groups on the polymer backbone that enable thepolymer to absorb aqueous fluids, such as body fluids.

SAPs are manufactured by known polymerization techniques, preferably bypolymerization in aqueous solution by gel polymerization. The productsof this polymerization process are aqueous polymer gels, i.e., SAPhydrogels, that are reduced in size to small particles by mechanicalforces, then dried using drying procedures and apparatus known in theart. The drying process is followed by pulverization of the resultingSAP particles to the desired particle size.

To improve the fluid absorption profile, SAP particles are optimizedwith respect to one or more of absorption capacity, absorption rate,acquisition time, gel strength, and/or permeability. Optimization allowsa reduction in the amount of cellulosic fiber in a hygienic article,which results in a thinner article. However, it is difficult toimpossible to maximize all of these absorption profile propertiessimultaneously.

One method of optimizing the fluid absorption profile of SAP particlesis to provide SAP particles of a predetermined particle sizedistribution. In particular, particles too small in size, afterabsorbing a fluid and swelling, and can block the absorption of furtherfluid. Particles too large in size have a reduced surface area whichdecreases the rate of absorption.

Therefore, the particle size distribution of SAP particles is such thatfluid permeability, absorption, and retention by the SAP particles ismaximized. Any subsequent process that reduces SAP particle size shouldbe avoided. In particular, fine-sized SAP particles are prone to gelblocking, which adversely affects absorption of an aqueous fluid by theSAP particles.

Invariably, a substantial amount of fine-sized SAP particles, i.e.,particles less than about 100 microns in size, are produced during SAPmanufacture. These fine-sized SAP particles adversely affect SAPperformance in a majority of practical applications, and are separatedfrom SAP particles of desired size. It is economically disadvantageousto discard the fine-sized SAP particles, therefore, the particles arerecycled into an early stage of the SAP manufacturing process. Thisrecycling step also is costly because the previously dried SAP fines arerehydrated, then dried again. In addition, each SAP manufacturingprocess generates additional SAP fines, so the recycling process isconstant.

The present invention is directed to abating the problem of fine-sizedSAP particles generated during the manufacturing process. The problem ofSAP fines is abated because of the polymer and/or wax coating applied tothe surfaces of the SAP particles. More particularly, the present methodof manufacturing SAP particles substantially reduces the amount of SAPfines, and the SAP particles maintain the conflicting fluid absorptionproperties of a high centrifuge retention capacity (CRC), absorptionunder load (AUL), and an excellent permeability.

In order to use an increased amount of SAP particles, and a decreasedamount of cellulose, in personal care products, it is important tomaintain a high liquid permeability. In particular, the permeability ofan SAP particle hydrogel layer formed by swelling in the presence of abody fluid is very important to overcome the problem of leakage from theproduct. Fine-sized SAP particles adversely affect permeability, and alack of permeability directly impacts the ability of SAP particle layersto acquire and distribute body fluids.

Numerous approaches to reduce the amount of fine-sized SAP particlesgenerated during the manufacturing process have been proposed. Theseprior methods include using additional crosslinking agent orpolymerization initiator near the end of SAP manufacture. Anotherapproach is disclosed in Japanese Patent Publication 622411972, whichdiscloses coating SAP particle surfaces with a polymer emulsion, thenover-coating with a coarse emulsion. The second coating provides awaterproof coating, i.e., a low water penetration of 2.1%.

Other patents directed to reducing the amount of, or adverse effects of,SAP fines include U.S. Pat. No. 5,478,879; EP 0 401 044; U.S. Pat. No.4,970,267; and U.S. Pat. No. 5,064,582. However, these and otherapproaches continue to generate a significant amount of SAP particlestoo small in size for current market applications. Using theseapproaches, substantial amounts of SAP fines still are generated duringsubsequent processing steps when the SAP particles, before or aftersurface crosslinking, are ground and sieved to meet desired particlesize specifications.

In accordance with the present invention, optionally surface-crosslinkedSAP particles coated with a polymer and/or wax are disclosed. Thepresent SAP particles comprise a base polymer. The base polymer can be ahomopolymer or a copolymer. The identity of the base polymer is notlimited as long as the polymer is an anionic polymer, i.e., containspendant acid moieties, and is capable of swelling and absorbing at leastten times its weight in water, when in a neutralized form. Preferredbase polymers are crosslinked polymers having acid groups that are atleast partially in the form of a salt, generally an alkali metal orammonium salt.

The base polymer typically has at least about 25 mol % of the pendantacid moieties, i.e., carboxylic acid moieties, present in a neutralizedform. Preferably, the base polymer has greater than 25 and up to about100 mol %, and more preferably about 50 up to about 100 mol %, of thependant acid moieties present in a neutralized form. The base polymer;therefore, has a degree of neutralization (DN) of at least 25 to about100.

The SAP is a lightly crosslinked polymer capable of absorbing severaltimes its own weight in water and/or saline. SAP particles can be madeby any conventional process for preparing superabsorbent polymers andare well known to those skilled in the art. One process for preparingSAP particles is a solution polymerization method described in U.S. Pat.Nos. 4,076,663; 4,286,082; 4,654,039; and 5,145,906, each incorporatedherein by reference. Another process is an inverse suspensionpolymerization method described in U.S. Pat. Nos. 4,340,706; 4,491,930;4,666,975; 4,507,438; and 4,683,274, each incorporated herein byreference. Each of these processes yields a substantial amount offine-sized SAP particles that require recycling.

SAP particles are prepared from one or more monoethylenicallyunsaturated compound having at least one acid moiety, such as carboxyl,carboxylic acid anhydride, carboxylic acid salt, sulfuric acid, sulfuricacid salt, sulfonic acid, sulfonic acid salt, phosphoric acid,phosphoric acid salt, phosphonic acid, or phosphonic acid salt. SAPparticles preferably are prepared from one or more monoethylenicallyunsaturated, water-soluble carboxyl or carboxylic acid anhydridecontaining monomer, and the alkali metal and ammonium salts thereof,wherein these monomers preferably comprise 50 to 99.9 mole percent ofthe base polymer.

The SAP preferably is a lightly crosslinked acrylic resin, such aslightly crosslinked polyacrylic acid. The lightly crosslinked SAPtypically is prepared by polymerizing an acidic monomer containing anacyl moiety, e.g., acrylic acid, or a moiety capable of providing anacid group, i.e., acrylonitrile, in the presence of an internalcrosslinking agent, i.e., a polyfunctional organic compound. The SAP cancontain other copolymerizable units, i.e., other monoethylenicallyunsaturated comonomers, well known in the art, as long as the basepolymer is substantially, i.e., at least 10%, and preferably at least25%, acidic monomer units, e.g., (meth)acrylic acid. To achieve the fulladvantage of the present invention, the SAP contains at least 50%, andmore preferably, at least 75%, and up to 100%, acidic monomer units. Theother copolymerizable units can, for example, help improve thehydrophilicity of the SAP.

Ethylenically unsaturated carboxylic acid and carboxylic acid anhydridemonomers useful in an SAP include acrylic acid, methacrylic acid,ethacrylic acid, α-chloroacrylic acid, α-cyanoacrylic acid,β-methylacrylic acid (crotonic acid), α-phenylacrylic acid,β-acryloxypropionic acid, sorbic acid, α-chlorosorbic acid, angelicacid, cinnamic acid, p-chlorocinnamic acid, β-stearylacrylic acid,itaconic acid, citraconic acid, mesaconic acid, glutaconic acid,aconitic acid, maleic acid, fumaric acid, tricarboxyethylene, and maleicanhydride.

Ethylenically unsaturated sulfonic and phosphonic acid monomers includealiphatic or aromatic vinyl sulfonic acids, such as vinyl sulfonic acid,allyl sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid,acrylic and methacrylic sulfonic acids, such as sulfoethyl acrylate,sulfoethyl methacrylate, sulfopropyl acrylate, sulfopropyl methacrylate,2-hydroxy-3-methacryloxypropyl sulfonic acid,2-acrylamido-2-methylpropane sulfonic acid, vinyl phosphonic acid, allylphosphonic acid, and mixtures thereof.

Preferred monomers include acrylic acid, methacrylic acid, maleic acid,fumaric acid, maleic anhydride, and the sodium, potassium, and ammoniumsalts thereof. An especially preferred monomer is acrylic acid.

The SAP can contain additional monoethylenically unsaturated monomersthat do not bear a pendant acid group, but are copolymerizable withmonomers bearing acid groups. Such compounds include, for example, theamides and nitriles of monoethylenically unsaturated carboxylic acids,for example, acrylamide, methacrylamide, acrylonitrile, andmethacrylonitrile. Examples of other suitable comonomers include, butare not limited to, vinyl esters of saturated C₁₋₄ carboxylic acids,such as vinyl formate, vinyl acetate, and vinyl propionate; alkyl vinylethers having at least two carbon atoms in the alkyl group, for example,ethyl vinyl ether and butyl vinyl ether; esters of monoethylenicallyunsaturated C₃₋₁₈ alcohols and acrylic acid, methacrylic acid, or maleicacid; monoesters of maleic acid, for example, methyl hydrogen maleate;acrylic and methacrylic esters of alkoxylated monohydric saturatedalcohols, for example, alcohols having 10 to 25 carbon atoms reactedwith 2 to 200 moles of ethylene oxide and/or propylene oxide per mole ofalcohol; and monoacrylic esters and monomethacrylic esters ofpolyethylene glycol or polypropylene glycol, the molar masses (M_(n)) ofthe polyalkylene glycols being up to about 2,000, for example. Furthersuitable comonomers include, but are not limited to, styrene andalkyl-substituted styrenes, such as ethylstyrene and tert-butylstyrene,and 2-hydroxyethyl acrylate.

Polymerization of the acidic monomers, and any copolymerizable monomers,most commonly is performed by free radical processes in the presence ofa polyfunctional organic compound. The base polymers are internallycrosslinked to a sufficient extent such that the base polymer is waterinsoluble. Internal crosslinking, in part, serves to determine theabsorption capacity of the base polymer. For use in absorptionapplications, a base polymer is lightly crosslinked, i.e., has acrosslinking density of less than about 20%, preferably less than about10%, and most preferably about 0.01% to about 7%.

A crosslinking agent most preferably is used in an amount of less thanabout 7 wt %, and typically about 0.1 wt % to about 5 wt %, based on thetotal weight of monomers. Examples of crosslinking polyvinyl monomersinclude, but are not limited to, polyacrylic (or polymethacrylic) acidesters represented by the following formula (I), and bisacrylamidesrepresented by the following formula (II):

wherein X is ethylene, propylene, trimethylene, cyclohexyl,hexamethylene, 2-hydroxypropylene, —(CH₂CH₂O)CH₂CH₂—, or CH₃ CH₃

n and m are each an integer 5 to 40, and k is 1 or 2;

wherein 1 is 2 or 3.

The compounds of formula (I) are prepared by reacting polyols, such asethylene glycol, propylene glycol, trimethylolpropane, 1,6-hexanediol,glycerin, pentaerythritol, polyethylene glycol, or polypropylene glycol,with acrylic acid or methacrylic acid. The compounds of formula (II) areobtained by reacting polyalkylene polyamines, such as diethylenetriamineand triethylenetetramine, with acrylic acid.

Specific internal crosslinking agents include, but are not limited to,1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,3-butyleneglycol diacrylate, 1,3-butylene glycol dimethacrylate, diethylene glycoldiacrylate, diethylene glycol dimethacrylate, ethoxylated bisphenol Adiacrylate, ethoxylated bisphenol A dimethacrylate, ethylene glycoldimethacrylate, 1,6-hexanediol diacrylate, 1,6-hexanedioldimethacrylate, neopentyl glycol dimethacrylate, polyethylene glycoldiacrylate, polyethylene glycol dimethacrylate, triethylene glycoldiacrylate, triethylene glycol dimethacrylate, tripropylene glycoldiacrylate, tetraethylene glycol diacrylate, tetraethylene glycoldimethacrylate, dipentaerythritol pentaacrylate, pentaerythritoltetraacrylate, pentaerythritol triacrylate, trimethylolpropanetriacrylate, trimethylolpropane trimethacrylate,tris(2-hydroxyethyl)-isocyanurate triacrylate, ethoxylatedtrimethylolpropane triacrylate (ETMPTA), e.g., ETMPTA ethyoxylated with15 moles of ethylene oxide (EO) on average,tris(2-hydroxyethyl)isocyanurate trimethyacrylate, divinyl esters of apolycarboxylic acid, diallyl esters of a polycarboxylic acid, triallylterephthalate, diallyl maleate, diallyl fumarate,hexamethylenebismaleimide, trivinyl trimellitate, divinyl adipate,diallyl succinate, a divinyl ether of ethylene glycol, cyclopentadienediacrylate, a tetraallyl ammonium halide, divinyl benzene, divinylether, diallyl phthalate, or mixtures thereof. Especially preferredinternal crosslinking agents are N,N′-methylenebisacrylamide,N,N′-methylenebismethacrylamide, ethylene glycol dimethacrylate, andtrimethylolpropane triacrylate.

The SAP can be any internally-crosslinked polymer having pendant acidmoieties that absorbs several times its weight of water in itsneutralized form. Examples of SAPs include, but are not limited to,polyacrylic acid, hydrolyzed starch-acrylonitrile graft copolymers,starch-acrylic acid graft copolymers, saponified vinyl acetate-acrylicester copolymers, hydrolyzed acrylonitrile copolymers, hydrolyzedacrylamide copolymers, ethylene-maleic anhydride copolymers,isobutylene-maleic anhydride copolymers, poly(vinylsulfonic acid),poly(vinylphosphonic acid), poly(vinylphosphoric acid),poly(vinylsulfuric acid), sulfonated polystyrene, poly(aspartic acid),poly(lacetic acid), and mixtures thereof. A preferred SAP is ahomopolymer or copolymer of acrylic acid or methacrylic acid.

The free radical polymerization is initiated by an initiator or byelectron beams acting on a polymerizable aqueous mixture. Polymerizationalso can be initiated in the absence of such initiators by the action ofhigh energy radiation in the presence of photoinitiators.

Useful polymerization initiators include, but are not limited to,compounds that decompose into free radicals under polymerizationconditions, for example, peroxides, hydroperoxides, persulfates, azocompounds, and redox catalysts. Water-soluble initiators are preferred.In some cases, mixtures of different polymerization initiators are used,for example, mixtures of hydrogen peroxide and sodium peroxodisulfate orpotassium peroxodisulfate. Mixtures of hydrogen peroxide and sodiumperoxodisulfate can be in any proportion.

Examples of suitable organic peroxides include, but are not limited to;acetylacetone peroxide, methyl ethyl-ketone peroxide, tert-butylhydroperoxide, cumene hydroperoxide, tert-amyl perpivalate, tert-butylperpivalate, tert-butyl perneohexanoate, tert-butyl perisobutyrate,tert-butyl per-2-ethylhexanoate, tert-butyl perisononanoate, tert-butylpermaleate, tert-butyl perbenzoate, di(2-ethylhexyl) peroxydicarbonate,dicyclohexyl peroxydicarbonate, di(4-tert-butylcyclohexyl)peroxydicarbonate, dimyristyl peroxydicarbonate, diacetylperoxydicarbonate, an allyl perester, cumyl peroxyneodecanoate,tert-butyl per-3,5,5-trimethyl-hexanoate, acetylcyclohexylsulfonylperoxide, dilauryl peroxide, dibenzoyl peroxide, and tert-amylperneodecanoate. Particularly suitable polymerization initiators arewater-soluble azo initiators, e.g., 2,2′-azobis(2-amidinopropane)dihydrochloride, 2,2′-azobis(N,N′-dimethylene)isobutyramidinedihydrochloride, 2-(carbamoylazo-isobutyronitrile,2,2′-azobis[2-(2′-imidazolin-2-yl)propane]dihydrochloride, and4,4′-azobis(4-cyanovaleric acid). The polymerization initiators areused, for example, in amounts of 0.01% to 5%, and preferably 0.05% to2%, by weight, based on the monomers to be polymerized.

Polymerization initiators also include redox catalysts. In redoxcatalysts, the oxidizing compound comprises at least one of theabove-specified per compounds, and the reducing component comprises, forexample, ascorbic acid, glucose, sorbose, ammonium or alkali metalbisulfite, sulfite, thiosulfate, hyposulfite, pyrosulfite, or sulfide,or a metal salt, such as iron (II) ions or sodiumhydroxymethylsulfoxylate. The reducing component of the redox catalystpreferably is ascorbic acid or sodium sulfite. Based on the amount ofmonomers used in the polymerization, about 3×10⁻⁶ to about 1 mol % ofthe reducing component of the redox catalyst system can be used, andabout 0.001 to about 5.0 mol % of the oxidizing component of the redoxcatalyst can be used, for example.

When polymerization is initiated using high energy radiation, theinitiator typically comprises a photoinitiator. Photoinitiators include,for example, α-splitters, H-abstracting systems, and azides. Examples ofsuch initiators include, but are not limited to, benzophenonederivatives, such as Michler's ketone; phenanthrene derivatives;fluorene derivatives; anthraquinone derivatives; thioxanthonederivatives; coumarin derivatives; benzoin ethers and derivativesthereof; azo compounds, such as the above-mentioned free-radicalformers, substituted hexaarylbisimidazoles, acylphosphine oxides; ormixtures thereof.

Examples of azides include, but are not limited to,2-(N,N-dimethylamino)ethyl 4-azidocinnamate, 2-(N,N-dimethylamino)ethyl4-azidonaphthyl ketone, 2-(N,N-dimethylamino)ethyl 4-azidobenzoate,5-azido-1-naphthyl 2′-(N,N-dimethylamino)ethyl sulfone,N-(4-sulfonylazidophenyl)maleimide, N-acetyl-4-sulfonylazidoaniline,4-sulfonyl-azidoaniline, 4-azidoaniline, 4-azidophenacyl bromide,p-azidobenzoic acid, 2,6-bis(p-azidobenzylidene)cyclohexanone, and2,6-bis(p-azidobenzylidene)-4-methylcyclohexanone. Photoinitiatorscustomarily are used, if at all, in amounts of about 0.01% to about 5%,by weight of the monomers to be polymerized.

As previously stated, the base polymer is partially neutralized. Thedegree of neutralization preferably is greater than about 25 mol %, morepreferably about 50 to about 100 mol %, most preferably about 70 toabout 100 mol %, based on monomers containing acid groups.

Useful neutralizing agents for the base polymer include alkali metalbases, ammonia, and/or amines. Preferably, the neutralizing agentcomprises aqueous sodium hydroxide, aqueous potassium hydroxide, orlithium hydroxide. However, neutralization also can be achieved usingsodium carbonate, sodium bicarbonate, potassium carbonate, or potassiumbicarbonate, or other carbonates or bicarbonates, as a solid or as asolution. Primary, secondary, and/or tertiary amines can be used toneutralize the base polymer.

Neutralization of the SAP can be performed before, during, or afterpolymerization of the monomers in a suitable apparatus for this purpose.The neutralization is performed, for example, directly in a kneader usedfor polymerization of the monomers.

Polymerization of an aqueous monomer solution, i.e., gel polymerization,is a preferred manufacturing method. In this method, a 10% to 70%, byweight, aqueous solution of the monomers, including the internalcrosslinking agent, is neutralized in the presence of a free radicalinitiator. The solution polymerization is performed at 0° C. to 150° C.,preferably at 10° C. to 100° C., and at atmospheric, superatmospheric,or reduced pressure. The polymerization also can be conducted under aprotective gas atmosphere, preferably under nitrogen.

After polymerization, the resulting SAP hydrogel is comminuted anddried, then the resulting dry SAP particles are ground and classified toa predetermined size for an optimum fluid absorption profile. The SAPparticles then are surface crosslinked. Surface crosslinking of the SAPparticles is optional, however, the SAP particles typically are surfacecrosslinked. As discussed more fully hereafter, the SAP particles firstcan be surface crosslinked, then coated with the film-forming polymerand/or wax. Alternatively, surface crosslinking can be performedsimultaneously with applying the film-forming polymer and/or wax to theSAP particles. When a surface crosslinking step is omitted, thefilm-forming polymer and/or wax is applied prior to or during a dryingstep after an SAP hydrogel is comminuted into particles.

In the optional surface crosslinking process, a multifunctional compoundcapable of reacting with the functional groups of the SAP is applied tothe surfaces of the SAP particles, preferably using an aqueous solution.The aqueous solution also can contain water-miscible organic solvents,like an alcohol, such as methanol, ethanol, or i-propanol; a polyol,like ethylene glycol or propylene glycol; or acetone.

A solution of a surface crosslinking agent is applied to the SAPparticles in an amount to wet predominantly only the outer surfaces ofthe SAP particles, either before, during, or after application of thepolymer and/or wax. Surface crosslinking and drying of the SAP particlesthen is performed, preferably by heating at least the wetted surfaces ofthe SAP particles.

Typically, the SAP particles are surface treated with a solution of asurface crosslinking agent containing about 0.01% to about 4%, byweight, surface crosslinking agent, and preferably about 0.4% to about2%, by weight, surface crosslinking agent, in a suitable solvent. Thesolution can be applied as a fine spray onto the surfaces of freelytumbling SAP particles at a ratio of about 1:0.01 to about 1:0.5 partsby weight SAP particles to solution of surface crosslinking agent. Thesurface crosslinking agent, if present at all, is present in an amountof 0.001% to about 5%, by weight of the SAP particles, and preferably0.001% to about 0.5% by weight. To achieve the full advantage of thepresent invention, the surface crosslinking agent is present in anamount of about 0.001% to about 0.1%, by weight of the SAP particles.

Surface crosslinking and drying of the SAP particles are achieved byheating the surface-treated SAP particles at a suitable temperature,e.g., about 70° C. to about 150° C., and preferably about 105° C. toabout 120° C. Suitable surface crosslinking agents are capable ofreacting with acid moieties and crosslinking polymers at the surfaces ofthe SAP particles.

Nonlimiting examples of suitable surface crosslinking agents include,but are not limited to, an alkylene carbonate, such as ethylenecarbonate or propylene carbonate; a polyaziridine, such as2,2-bishydroxymethyl butanol tris[3-(1-aziridine propionate] orbis-N-aziridinomethane; a haloepoxy, such as epichlorohydrin; apolyisocyanate, such as 2,4-toluene diisocyanate; a di- or polyglycidylcompound, such as diglycidyl phosphonates, ethylene glycol diglycidylether, or bischlorohydrin ethers of polyalkylene glycols; alkoxysilylcompounds; polyols such as ethylene glycol, 1,2-propanediol,1,4-butanediol, glycerol, methyltriglycol, polyethylene glycols havingan average molecular weight M_(w) of 200-10,000, di- and polyglycerol,pentaerythritol, sorbitol, the ethoxylates of these polyols and theiresters with carboxylic acids or carbonic acid, such as ethylenecarbonate or propylene carbonate; carbonic acid derivatives, such asurea, thiourea, guanidine, dicyandiamide, 2-oxazolidinone and itsderivatives, bisoxazoline, polyoxazolines, di- and polyisocyanates; di-and poly-N-methylol compounds, such asmethylenebis(N-methylolmethacrylamide) or melamine-formaldehyde resins;compounds having two or more blocked isocyanate groups, such astrimethylhexamethylene diisocyanate blocked with2,2,3,6-tetramethylpiperidin-4-one; and other surface crosslinkingagents known to persons skilled in the art.

A solution of the surface crosslinking agent is applied to the surfacesof the SAP particles simultaneously with, or before or after, adispersion containing the film-forming polymer or wax is applied to thesurfaces of the SAP particles. The film-forming polymer or wax typicallyis applied prior to completion of the surface crosslinking step. Inembodiments wherein a film-forming polymer and/or wax is applied to SAPparticles that will not be surface crosslinked, the polymer and/or waxis applied after chopping of the SAP hydrogel and prior to completelydrying the SAP particles.

A dispersion or slurry containing the film-forming polymer or waxcomprises about 5% to about 75%, by weight, of the polymer or wax in asuitable carrier. The polymer or wax is dispersed in a sufficient amountof a carrier to allow the polymer or wax to be readily and homogeneouslyapplied to the surfaces of the SAP particles. The carrier for thepolymer or wax can be, but is not limited to, water, an alcohol, or aglycol, such as methanol, ethanol, ethylene glycol, or propylene glycol,and mixtures thereof. Often, the polymer or wax is applied as anemulsion containing the polymer or wax, water, optional organicsolvents, emulsifiers, and other ingredients typically used in thepreparation of emulsions.

The nonreactive, film-forming polymer or wax applied to the SAPparticles is in the form of an aqueous dispersion, emulsion, orsuspension. However, the polymer or wax also can be applied from asolution wherein the polymer or wax is dissolved in an organic solventor a mixture of water and an organic water-miscible solvent.

Additional suitable organic carriers for the film-forming polymer or waxinclude, but are not limited to, aliphatic and aromatic hydrocarbons,alcohols, ethers, esters, and ketones, for example, n-hexane,cyclohexane, toluene, xylene, methanol, ethanol, i-propanol, ethyleneglycol, 1,2-propanediol, glycerol, diethyl ether, methyltriglycol, apolyethylene glycol having an average molecular weight (Mw) of200-10,000, ethyl acetate, n-butyl acetate, acetone, 2-butanone, andmixtures thereof.

The amount of polymer and/or wax applied to the surfaces of the SAPparticles is sufficient to coat the SAP particle surfaces, but notadversely affect the absorption profile of the SAP particles.Accordingly, the amount of polymer applied to the surfaces of the SAPparticles is less than about 0.1% of the weight of the SAP particles.Preferably, the amount of polymer applied to the surfaces of the SAPparticles is about 0.01% to about 0.08%, by weight of the SAP particles.To achieve the full advantage of the present invention, the amount ofpolymer applied to the SAP particle surfaces is about 0.02% to about0.07%, by weight of the SAP particles. The amount of wax applied to thesurfaces of the SAP particles is less than about 0.05%, and preferablyabout 0.01% to less than 0.05%, by weight of the SAP particles.

The polymer and/or wax applied to surfaces of the SAP particles allowfor the collection and aggregation of SAP fines generated during SAPmanufacture. Accordingly, the problem of fine-sized SAP particlegeneration is substantially reduced. Furthermore, coated SAP particlesof the present invention are not covered with a closed polymer and/orwax film. Rather, the surfaces of SAP particles coated with the polymerand/or wax retain pores of sufficient size and sufficient number suchthat no adverse effect on the absorption properties of the SAP particlesoccurs. If the SAP particles are coated with too great of a quantity ofpolymer or wax, then the number and size of the pores on the particlesdecrease and an undesirable impairment of SAP absorption propertyresults. In addition, if the amount of polymer or wax applied to surfaceof the SAP particles is too great, a friable film having a tendency toflake off the SAP particles results. The above-listed amounts of polymeror wax permits the aggregation of fine-sized SAP particles, withoutadversely affecting the absorption of a fluid by the SAP particles.

After heating the coated SAP particles to form a coating on the SAPparticle surfaces, the resulting polymer and/or wax coating on the SAPparticle has sufficient adhesive forces to aggregate fine-sized SAPparticles to one another or to a larger SAP particle. The adhesive forceis sufficient such that SAP fines are not regenerated during subsequentSAP processing or after the SAP particles absorb a fluid and swell.

A film-forming polymer useful in the present invention is different froman SAP, i.e., the polymer does not absorb several times its weight inwater and swell. The applied polymer is water insoluble and isnonreactive, i.e., contains no reactive group which can react with thecarboxyl groups on the surface of the SAP particles. Suitable polymersfor use in the present invention have a glass transition temperature,T_(g), of less than about 30° C., preferably less than about 25° C., andmore preferably less than about 20° C., and down to at −20° C., forexample. The glass transition temperature is the temperature at which anamorphous material, such as a polymer, changes from a brittle vitreousstate to a plastic state. The T_(g) of a polymer is readily determinedby persons skilled in the art using standard techniques. Preferredpolymers form tackless polymer films between about 0° C. and about 80°C.

Useful polymers include homo- and copolymers of vinyl esters, inparticular vinyl acetate homopolymers and vinyl acetate copolymers withethylene, acrylates, maleic acid esters, vinylamides, and/or othervinylacyl derivatives. Homo- and copolymers of acrylic and methacrylicacid esters, such as copolymers of methyl methacrylate, n-butylacrylate, or 2-ethylhexyl acrylate, also are useful.

Copolymers based on vinyl esters, acrylic acid esters, and methacrylicacid esters comprise comonomers, for example, styrene, butadiene, avinylamide, an olefinically unsaturated carboxylic acid or derivativesthereof, a vinylphosphonic acid or derivative thereof, or a polyglycolester of unsaturated acids. Examples of vinylamides include, but are notlimited to, N-vinylformamide, N-vinyl-N-methylacetamide, andN-vinylpyrrolidone.

Examples of olefinically unsaturated carboxylic acids are, for example,acrylic acid, methacrylic acid, itaconic acid, and maleic acid. Examplesof derivatives of these olefinically unsaturated carboxylic acids are,for example, amides, such as (meth)acrylamide,N-tert-butyl(meth)acrylamide, and N-isopropyl(meth)acrylamide, and theN-methylolamides or ethers of N-methylolamides, hemiamides, and imidesof aliphatic amines, as well as acrylonitrile. Examples of derivativesof vinylphosphonic acid are, for example, the mono- and diesters ofC₁-C₁₈ alcohols, for example, the methyl, propyl, or stearyl esters.

Glycol esters of unsaturated acids include hydroxyethyl(meth)acrylate oresters of acrylic and methacrylic acid with polyalkylene oxide compoundsof the general formula

wherein X¹ is hydrogen or methyl, n is 0 to 50, and R is an alkyl,alkaryl, or cycloalkyl C₁-C₂₄ radical, for example, octylphenyl,dodecyl, or nonylphenyl.

Other film-forming polymers are polyacetals, i.e., reaction products ofpolyvinyl alcohols with aldehydes, such as, for example, butyraldehyde;polyurethane polymers prepared from polyhydric alcohols and isocyanates,for example, prepared from polyester and/or polyether diols and, forexample, toluene-2,4- or 2,6-diisocyanate, methylene-4,4-di(phenylisocyanate), or hexamethylene diisocyanate; polyureas, i.e., polymersprepared from diamines and diisocyanates or by polycondensation ofdiamines with carbon dioxide, phosgene, carboxylic acid esters (forexample, activated diphenyl carbonates), or urea, or by reaction ofdiisocyanates with water; polysiloxanes, i.e., lineardimethylpolysiloxane having end groups blocked in different ways;polyamides and copolyamides; polyesters, i.e., polymers prepared byring-opening polymerization of lactones or by condensation ofhydroxycarboxylic acids or diols and dicarboxylic acid derivatives;epoxy resins prepared from polyepoxides by addition reactions withsuitable curing agents or by polymerization by way of epoxide groups;polycarbonates prepared by reaction of di-glycols or bisphenols withphosgene or carbonic acid diesters in condensation ortransesterification reactions; and mixtures thereof.

Preferred polymers are homo- and co-polymers of acrylic acid esters andmethacrylic acid esters, and polymers based on polyacetals. Mixtures oftwo or more polymers also can be used. The mixture ratios arenoncritical and are judiciously determined by persons skilled in the artto fit the particular circumstances.

The SAP particles also can be coated with a water-insoluble, nonreactivewax to substantially reduce the amount of fine-sized SAP particlesduring SAP manufacture. Useful waxes have a melting point of about 30°C. to about 180° C., preferably about 40° C. to about 180° C. To achievethe full advantage of the present invention, the wax has a melting pointof about 40° C. to about 170° C. Preferred waxes form tackless films upto at least 80° C. Useful waxes also are nonreactive, i.e., they have noreactive groups capable of reacting with carboxyl groups on the surfaceof the SAP particles.

The following are nonlimiting examples of waxes useful in the presentinvention. In particular, examples of waxes useful in the presentinvention include natural waxes, modified natural waxes, semisyntheticwaxes, and synthetic waxes. Examples of natural waxes are plant waxes oranimal waxes. Plant waxes include, but are not limited to, carnauba wax,candelilla wax, ouricuri wax, sugarcane wax, and retamo wax. Examples ofanimal waxes include, but are not limited to, insect waxes, e.g.,beeswax, ghedda wax, shellac wax, and wool wax. Further examples ofnatural waxes are fossil waxes, such as petroleum waxes, brown coal(lignite), and peat waxes, e.g., ozokerite, tank-bottom wax, and crudemontan wax. Examples of modified natural waxes are waxes obtained byrefining, for example, the macro- and microcrystalline paraffin waxesrecovered from petroleum distillates or distillation residues, orchemically modified waxes, for example, double-bleached crude montanwax. Examples of semisynthetic waxes include, but are not limited to,acid waxes and ester waxes which can be prepared from montan wax, waxacids which can be prepared by paraffin oxidation, and alcohol waxes andamide waxes. Examples of synthetic waxes include, but are not limitedto, hydrocarbon waxes, such as polyolefin waxes and Fischer Tropschwaxes, and synthetic waxes containing oxygen-functional groups. Examplesof synthetic waxes containing oxygen-functional groups are acid waxesformed by oxidation of synthetic hydrocarbon waxes or bycopolymerization or telomerization of olefins with unsaturatedcarboxylic acids, ester waxes obtained by esterifying synthetic waxacids with synthetic alcohols and by copolymerizing olefins withunsaturated esters, for example, vinyl acetate, alcohol waxes preparedby oxo synthesis followed by hydrogenation and by hydrogenation ofsynthetic fatty acids, and amide waxes obtained by reacting syntheticacids with amines. Examples of waxes obtained by oxidation of synthetichydrocarbon waxes are oxidation products of polyethylene waxes.Preferred waxes for use in accordance with the present invention arerefined (i.e., deresinified or bleached) montan waxes and polyolefinwaxes.

Particularly preferred waxes for use in accordance with the presentinvention are polyolefin waxes, such as polyethylene waxes (highpressure polyethylene waxes, low pressure polyethylene waxes, anddegradation polyethylene waxes), oxidation products of thesepolyethylene waxes, waxes based on ethylene-α-olefin copolymers, waxesbased on ethylene-vinyl acetate copolymers, waxes based onethylene-styrene copolymers, waxes based on ethylene-acrylic acidcopolymers, and waxes based on wax mixtures of polyethylene waxes withpoly(tetrafluoroethylene) waxes.

Mixtures of two or more waxes also can be used. The mixture ratios arenoncritical and are judiciously determined by persons skilled in the artaccording to the particular circumstances. In addition, a mixture of afilm-forming polymer and a wax can be used to coat the SAP particles.

In accordance with the present invention, the polymer and/or wax isapplied to the SAP particles in a manner such that the polymer and/orwax is uniformly distributed on the surfaces of the SAP particles. Anyknown method for applying a liquid to a solid can be used, preferably bydispersing a coating liquid into fine droplets, for example, by use of apressurized nozzle or a rotating disc. Uniform coating of the SAPparticles can be achieved in a high intensity mechanical mixer or afluidized mixer which suspends the SAP particles in a turbulent gasstream. Methods for the dispersion of a liquid onto the surfaces of SAPparticles are known in the art, see, for example, U.S. Pat. No.4,734,478, incorporated herein by reference.

Our method of coating the SAP particles is applying the polymer and/orwax and a surface crosslinking agent (if used) simultaneously.Individual components preferably are applied via separate nozzles toavoid any interactions before application to the surfaces of the SAPparticles. Another method of coating the base polymer is a sequentialaddition of the components. The preferred method is a simultaneousapplication of a surface crosslinking agent and application of thepolymer and/or wax. The resulting coated SAP particles then are heatedat about 70° C. to about 175° C. for sufficient time, e.g., about 5 toabout 90 minutes, to cure the polymer and/or wax coating.

In accordance with the invention, the present methods substantiallyreduce the amount of SAP fines generated during SAP manufacture. Inparticular, the present methods generate less than 2%, and preferablyless than 1%, by weight, particles having a particle size of 100 micronsor less. More preferably, the methods generate less than 0.5%, byweight, SAP particles having a particle size of 100 microns or less.Accordingly, recycling of SAP fines is substantially reduced, whichprovides cost savings and improves final SAP performance.

In addition to the film-forming polymer and/or wax, the coating on theSAP particles can include an optional clay. The clay is applied to theSAP particles when the polymer, wax, or surface-crosslinking agent isapplied to the SAP particles. The clay can be applied alone or inadmixture with the polymer, wax, or optional surface-crosslinking agent.

A clay is present in the coating to provide the benefit of an additionalingredient to assist in agglomerating fine-sized SAP particles.Therefore, a clay is present, if at all, in an amount of about 0.1% toabout 1.5%, and preferably about 0.25% to about 1.25%, by weight of theSAP particles. To achieve the full advantage of the present invention,the clay is present in an amount of about 0.4% to about 1.1%, by weightof the SAP particles.

A clay useful in the present invention typically is a nonswelling clay.However, swelling clays also can be used. Suitable nonswelling claysinclude, without limitation, kaolin minerals (including kaolinite,dickite, and nacrite), serpentine minerals, mica minerals (includingillite), chlorite minerals, sepolite, palygorskite, bauxite, andmixtures thereof. Examples of useful swelling clays include, but are notlimited to, montmorillonite, i.e., bentonite, beidelite, hectorite,saponite, nontronite, sauconite, and laponite.

To demonstrate the unexpected advantages provided by the method of thepresent invention, polymer or wax-coated SAP particles were prepared andtested for centrifuge retention capacity (CRC, g/g), absorbency underload (e.g., AUL 0.7 psi, g/g), and particle size distribution. Thesetests were performed using the following procedures.

Centrifuge Retention Capacity (CRC)

This test determines the free swelling capacity of a hydrogel-formingpolymer. In this method, 0.2000±0.0050 g of dry SAP particles of sizefraction 106 to 850 μm are inserted into a teabag. The teabag is placedin saline solution (i.e., 0.9 wt % aqueous sodium chloride) for 30minutes (at least 0.83 l (liter) saline solution/1 g polymer). Then, theteabag is centrifuged for 3 minutes at 250 G. The absorbed quantity ofsaline solution is determined by measuring the weight of the teabag.

Absorbency Under Load (AUL)

This procedure is disclosed in WO 00/62825, pages 22-23, incorporatedherein by reference, using a 230 gram weight for an AUL (0.70 psi). AUL(0.01 psi) and AUL (0.9 psi) are determined using the same procedureusing an appropriate weight, e.g., a 317 gram weight for an AUL (0.9psi).

Particle Size Distribution (PSD)

Particle size distribution is determined as set forth in U.S. Pat. No.5,061,259, incorporated herein by reference. In summary, a sample of SAPparticles is added to the top of a series of stacked sieves. The sievesare mechanically shaken for a predetermined time, then the amount of SAPparticles on each sieve is weighed. The percent of SAP particles on eachsieve is calculated from the initial sample weight of the SAP sample.

Example 1

Base SAP particles: CRC = 40 g/g PSD: >850 μm: 0.1% 500-850 μm: 33.8% 300-500 μm: 32.6%  150-300 μm: 26.5%  106-150 μm: 5.7%  45-106 μm: 1.3%<45 μm:  0.0%. The SAP particles¹⁾ (40 kg) were coated with 2 kg ofdeionized water, 2 kg of propylene glycol, 0.025 kg of polymer²⁾, and0.04 kg of ethylene glycol digly- cidyl ether (EGDGE). The SAP particlesthen were cured at 120° C. for 60 minutes. Finished product performance:CRC = 30.3 g/g PSD: >850 μm: 0.3% 500-850 μm: 36.0%  300-500 μm: 32.8% 150-300 μm: 25.8%  106-150 μm: 4.1%  45-106 μm: 1.0% <45 μm: 0.0%¹⁾The SAP particles were a crosslinked polyacrylic acid having a 75%neutralization; and²⁾Added as an emulsion containing 50.5%, by weight of a methylmethacrylate/butyl acrylate copolymer, to provide 0.025 kg of thepolymer.

Comparative Example 1

Base SAP particles: CRC = 40 g/g PSD: >850 μm: 0.1% 500-850 μm: 33.8% 300-500 μm: 32.6%  150-300 μm: 26.5%  106-150 μm; 5.7%  45-106 μm: 1.3%<45 μm: 0.0% The SAP particles¹⁾ (40 kg) were coated with 2 kg ofdeionized water, 2 kg of propylene glycol, and 0.04 kg of EGDGE. The SAPparticles then were cured at 120° C. for 60 minutes. Finished productperformance: CRC = 30.1 g/g AUL (0.7 psi) = 17.5 g/g PSD: >850 μm: 1.2%500-850 μm: 2.92%  300-500 μm: 28.2%  150-300 μm: 24.2%  106-150 μm;5.8%  45-106 μm: 9.3% <45 μm: 2.1%

Example 1 and Comparative Example 1 show that coating SAP particles witha polymer substantially reduces the amount of fine-sized SAP particlesgenerated during the surface crosslinking process, without adverselyaffecting fluid absorption properties.

A series of polymers and a wax were tested for an ability to reduce theamount of fine-sized SAP particles generated during SAP manufacture.Three of the four polymers were styrene/butadiene copolymers ofdifferent concentrations. The fourth polymer was a methylmethacrylate/butyl acrylate (MMA/BA) copolymer. The objective of thetest was to minimize the amount 106 micron and smaller SAP particlesgenerated during the surface crosslinking process. Polymer 1: Solidcontent = 50.5% MMA/BA copolymer; Polymer 2: Solid content = 70.8%styrene/butadiene copolymer; Polymer 3: Solid content = 50.8%styrene/butadiene copolymer; Polymer 4: Solid content = 52.5%styrene/butadiene copolymer.

Example 2

To polyacrylic acid SAP particles (DN=75) was added 4.2% (by weight,based on the weight of the SAP particles) of a mixture of a 1/1 ratio ofdeionized (DI) water and propylene glycol that further contained 0.1%Polymer 1 latex. The mixture was applied by spraying onto 1,000 grams ofSAP particles fluidized in a Lodige mixer. After applying Polymer 1, theSAP particles were heated to 120° C. and held for one hour. The initialSAP particles polymer had a 106 micron and smaller particle content of20%. After application of Polymer 1, the particle distribution wasmeasured and found to have 0% of particles having a size less than 106micron. The CRC of the coated SAP particles was 32.7 g/g and the SAULwas 62 (0.01 AUL=50 g/g and 0.9 AUL=12 g/g). Target properties of SAPparticles without added Polymer 1 are 30 g/g CRC and 64 SAUL. Thefinished product whiteness was 36.

Example 3

To polyacrylic acid SAP particles (DN=75) was added 4.2% (by weightbased on the weight of the SAP particles) of a mixture of a 1/1 ratio ofDI water and propylene glycol that further contained 0.1% EGDGE, 0.05%Polymer 1, and 1% of kaolin clay. The mixture first was sheared in aWaring Blender at the “high speed” condition for three minutes todisperse Polymer 1 more uniformly. The mixture then was applied byspraying onto 1,000 grams of SAP particles that were fluidized in aLodige mixer. After applying the mixture, the SAP particles were heatedto 120° C. and held for one hour. After application of thepolymer/kaolin clay coating, the mean PSD was measured and determined tobe 43% higher than the control (330 μm versus 281 μm) indicating a shiftin the particle distribution (PSD) towards the coarse end and areduction of the amount of fine-sized SAP particles.

Example 4

To polyacrylic acid SAP particles (DN=75) was added a 4% mixture (byweight based on the weight of the SAP particles) of a 3/1 ratio of DIwater and propylene glycol that also contains 4% EGDGE, 0.058%micronized polyethylene wax, and 1% of kaolin clay. This mixture wassheared in a Waring Blender at the “high speed” condition for threeminutes to disperse the wax more uniformly. The mixture then was appliedby spraying onto 1,000 grams of SAP particles fluidized in a Lodigemixer. After applying the mixture, the SAP particles were heated to 120°C. and held for one hour. The initial SAP particles had a less than 45micron content of 3.5%. After wax application, the particle distributionwas measured and found to have 2% of the particles less than 45 microns.

The SAP particles of the present invention are useful as absorbents forwater and other aqueous fluids, and can be used as an absorbentcomponent in hygiene articles, such as diapers, tampons, and sanitarynapkins. The SAP particles also can be used in the followingapplications, for example: storage, packaging, transportation as apackaging material for water-sensitive articles, for example, flowertransportation, and shock protection; food sector for transportation offish and fresh meat, and the absorption of water and blood in fresh fishand meat packs; water treatment, waste treatment and water removal;cleaning; and agricultural industry in irrigation, retention ofmeltwater and dew precipitates, and as a composting additive.

Particularly preferred applications for the present SAP particlesinclude medical uses (wound plaster, water-absorbent material for burndressings or for other weeping wounds, rapid dressings for injuries,rapid uptake of body fluid exudates for later analytical and diagnosticpurposes), cosmetics, carrier material for pharmaceuticals andmedicaments, rheumatic plaster, ultrasound gel, cooling gel, thickenersfor oil/water or water/oil emulsions, textile (gloves, sportswear,moisture regulation in textiles, shoe inserts, synthetic fabrics),hydrophilicization of hydrophobic surfaces, chemical process industryapplications (catalyst for organic reactions, immobilization of largefunctional molecules (enzymes), heat storage media, filtration aids,hydrophilic component in polymer laminates, dispersants, liquefiers),and building construction (sealing materials, systems or films thatself-seal in the presence of moisture, and fine-pore formers in sinteredbuilding materials or ceramics).

The present invention also provides for use of the SAP particles in anabsorption core of hygienic articles. Hygiene articles include, but arenot limited to, incontinence pads and incontinence briefs for adults,diapers for infants, catamenial devices, bandages, and similar articlesuseful for absorbing body fluids.

Hygiene articles, like diapers, comprise (a) a liquid pervious topsheet;(b) a liquid impervious backsheet; (c) a core positioned between (a) and(b) and comprising 10% to 100% by weight of the present SAP particles,and 0% to 90% by weight of a hydrophilic fiber material; (d) optionallya tissue layer positioned directly above and below said core (c); and(e) optionally an acquisition layer positioned between (a) and (c).

Obviously, many modifications and variations of the invention ashereinbefore set forth can be made without departing from the spirit andscope thereof and, therefore, only such limitations should be imposed asare indicated by the appended claims.

1. A superabsorbent polymer particle comprising a base polymer having asurface coating comprising less than 0.1%, by weight of the particle, ofa nonreactive, film-forming polymer; less than 0.05%, by weight of theparticle, of a wax; or both.
 2. The superabsorbent polymer particle ofclaim 1 wherein the particle is surface crosslinked.
 3. Thesuperabsorbent polymer particle of claim 1 wherein the base polymercontains a plurality of pendant neutralized and un-neutralizedcarboxylic acid groups.
 4. The superabsorbent polymer particle of claim1 wherein the base polymer has a degree of neutralization from about 25to about
 100. 5. The superabsorbent polymer particle of claim 1 whereinthe base polymer comprises acrylic acid, methacrylic acid, or a mixturethereof.
 6. The superabsorbent polymer particle of claim 1 wherein thepolymer is present on surfaces of the base polymer in an amount of about0.01% to about 0.08%, by weight of the particle.
 7. The superabsorbentpolymer particle of claim 1 wherein the polymer is present on surfacesof the base polymer in an amount of about 0.02% to about 0.07%, byweight of the particle.
 8. The superabsorbent polymer particle of claim1 wherein the wax is present on surfaces of the base polymer in anamount of about 0.01% to less than 0.05%, by weight of the particle. 9.The superabsorbent polymer particle of claim 1 wherein the coatingcomprises a film-forming polymer.
 10. The superabsorbent polymerparticle of claim 9 wherein the film-forming polymer comprise ahomopolymer or a copolymer of a vinyl ester, an acrylic acid ester, amethacrylic acid ester, a polyacetal, a polyurethane, a polysiloxane, apolyester, an epoxy resin, a polycarbonate, or mixtures thereof.
 11. Thesuperabsorbent polymer particle of claim 1 wherein the film-formingpolymer has a glass transition temperature of less than 30° C.
 12. Thesuperabsorbent polymer particle of claim 1 wherein the coating comprisesa wax.
 13. The superabsorbent polymer particle of claim 12 wherein thewax is selected from the group consisting of a natural wax, a modifiednatural wax, a semisynthetic wax, a synthetic wax, and mixtures thereof.14. The superabsorbent polymer particle of claim 12 wherein the wax isselected from the group consisting of a polyolefin wax, a montan wax, afossil wax, a peat wax, a micro- or macrocrystalline paraffin wax, anacid wax, an ester wax, an alcohol wax, an amide wax, a plant wax, ananimal wax, and mixtures thereof.
 15. The superabsorbent polymerparticle of claim 1 wherein the surface coating further comprises about0.1% to about 1.5%, by weight of the particle, of a clay.
 16. Thesuperabsorbent polymer particle of claim 1 wherein the base polymercomprises a polyacrylic acid.
 17. The superabsorbent polymer particle ofclaim 16 wherein the film-forming polymer comprises a styrene/butadienecopolymer, a methyl methacrylate/butyl acrylate copolymer, or a mixturethereof.
 18. The superabsorbent polymer particle of claim 16 wherein thewax comprises a micronized polyethylene wax, a montan wax, apolyethylene wax, or mixtures thereof.
 19. The superabsorbent polymerparticle of claim 16 wherein the base polymer is surface crosslinked.20. A method of preparing a superabsorbent polymer particle of claim 1comprising: (a) providing base polymer particles; (b) providing apolymer and a wax; (c) applying the polymer or the wax, or both, tosurfaces of the base polymer particles; (d) optionally applying asurface crosslinking agent to the surfaces of the base polymer; and (e)heating the coated base polymer resulting from steps (c) and (d) at asufficient temperature and for a sufficient time to provide a drypolymer or wax coating on the base polymer particles.
 21. The method ofclaim 20 wherein heating step (e) is performed at 70° C. to about 175°C. for about 5 minutes to about 90 minutes.
 22. The method of claim 20wherein step (c) is performed prior to step (d).
 23. The method of claim20 wherein step (c) is performed after step (d).
 24. The method of claim20 wherein step (c) and step (d) are performed simultaneously.
 25. Themethod of claim 20 wherein the superabsorbent polymer particles have aparticle size distribution wherein less than 2%, by weight, of theparticles have a particle size of 100 microns or less.
 26. The method ofclaim 20 wherein the superabsorbent polymer particles have afilm-forming polymer coating.
 27. The method of claim 20 wherein thesuperabsorbent polymer particles have a wax coating.
 28. The method ofclaim 20 further comprising the step of applying 0.1% to 1.5% of a clay,by weight of the base polymer particles, to surfaces of the base polymerparticles prior to step (e).
 29. A method of preparing superabsorbentpolymer particles having a film-forming polymer or wax coatingcomprising: (a) providing surface-crosslinked base polymer particles;(b) providing a polymer and a wax; (c) applying the polymer in an amountof less than 0.1% by weight of the base polymer particles, the wax in anamount of less than 0.05%, by weight of the base polymer particles, orboth, to surfaces of the base polymer particles; (d) heating the coatedbase polymer resulting from step (c) at a sufficient temperature and fora sufficient time to provide a dry polymer or wax coating on thesurface-crosslinked base polymer particles.
 30. The method of claim 29wherein the superabsorbent polymer particles have a particle sizedistribution wherein less than 2%, by weight, of the particles have aparticle size of 100 microns or less.
 31. The method of claim 29 whereinthe superabsorbent polymer particles have a film-forming polymercoating.
 32. The method of claim 29 wherein the superabsorbent polymerparticles have a wax coating.
 33. The method of claim 29 furthercomprising the step of applying 0.1% to 1.5% of a clay, by weight of thebase polymer particles, to surfaces of the base polymer particles priorto step (d).
 34. A hygienic article having a core, said core comprisingsuperabsorbent polymer particles of claim
 1. 35. The hygienic article ofclaim 34 wherein said article is selected from the group consisting of adiaper, an incontinence pad, an incontinence brief, a catamenial device,and a bandage.
 36. A hygiene article comprising (a) a liquid pervioustopsheet; (b) a liquid impervious backsheet; (c) a core positionedbetween (a) and (b) and comprising (i) 10% to 100% by weight of thesuperabsorbent polymer particles of claim 1 and (ii) 0% to 90% by weightof a hydrophilic fiber material; (d) optionally a tissue layerpositioned directly above and below said core (c); and (e) optionally anacquisition layer positioned between (a) and (c).
 37. The article ofclaim 36 wherein the hygiene article is a diaper.